In these days, seats for automobiles and other vehicles are often manufactured by furnishing polyurethane seat paddings each having a center seating portion and a pair of bank or raised portions on opposite sides thereof and covering the paddings with shells or covers. Some polyurethane seat paddings have uniform hardness throughout, but polyurethane seat paddings in which the bank portions are harder than the seating portion are more popular for their stable support to the human body and the improved aesthetic appearance of finished covers.
In the prior art, a number of methods are known for preparing polyurethane seat paddings having bank portions of higher hardness. Some typical prior art methods are described with reference to FIGS. 15 and 16. A polyurethane seat padding having a center seating portion and opposed bank portions of higher hardness than the seating portion is manufactured by a method as illustrated in FIG. 15 using a mold 11 which defines a cavity including a center region 12 corresponding to the seating portion and side regions 13 and 13 corresponding to the bank portions. Weir members 14 are provided at the boundary between the center region 12 and the side regions 13. Liquid A which is a foaming raw material for producing a low hardness foam is poured into the center region 12 and liquid B which is a foaming raw material for producing a high hardness foam is poured into the side regions 13. Then the liquids are expanded.
The method of FIG. 16 is a modified version of the method of FIG. 15 in that metallic or resinous partitions 15 and 15 are added to the mold 11. Metallic partitions 15 and 15 are integrally formed with the mold 11 such that the partitions stand on the bottom of the side regions 13 and 13. Alternatively resinous or partitions 15 and 15 of expanded resin are disposed in the mold 11 in a similar manner. Liquid A and B are poured to the inside and the outside of the partitions 15 and 15, respectively. In the case of metallic partitions, partitions are removed from the padding at the end of expansion process. In the case of resinous partitions, they are left in the padding because partions are joined to the polyurethane foam.
These methods for preparing polyurethane seat paddings having bank portions of higher hardness, however, have the following problems.
The padding manufactured by the method of FIG. 15 is shown in the cross section of FIG. 17. The bank portion 16 has high hardness throughout the section thereof. When one sits on this padding, the thigh comes in contact with the hard bank portion which is less comfortable to sit on. In addition, separation can occur at the interface between high and low hardness foams.
The padding manufactured by the method of FIG. 16 is shown in the cross sections of FIGS. 18 and 19. Where metallic partitions are used, the bank portion 16 at the top has a channel 17 from which the partition has been removed as shown in FIG. 18. Where partitions of expanded resin are used, the bank portion 16 at the top has the partition 15 embedded therein as shown in FIG. 19. When one sits on such a padding, the channel 17 or the partition 15 can be perceived as a foreign part which is less comfortable to sit on. Also failure can occur at the interface between high and low hardness foams.
Moreover, in the method of FIG. 16, part of liquids A and B can overrun the partitions 15 to move into the adjoining portions during expansion so that the interface between high and low hardness foams becomes indefinite, leading to a potential variation in quality. Since liquid B cannot fully spread to the rear side of the padding (which is depicted at 18 in FIGS. 18 and 19), problems arise with respect to the supporting ability and cover finishing.
Further, it is difficult to control the pouring of liquids A and B. Unless liquids A and B are poured in well controlled timing, either one of the liquids would flow over the partitions into the adjoining region in an unacceptable amount, failing to produce a quality padding.
Another method is disclosed in Balmisse et al., U.S. Pat. No. 4,923,746 or Japanese Patent Application Kokai No. 25008/1988. Polyurethane foam paddings are produced by placing vessels of expanded polystyrene against the opposed side walls of a mold cavity, pouring liquid B into the vessels and liquid A into the remaining cavity, and causing contraction of the vessels by external heating or exothermic reaction of the foams, thereby interconnecting the polyurethane foams of liquids A and B. This method is successful in improving comfortable seating, supporting ability, and cover finishing, but still has several drawbacks.
(1) A foaming raw material or liquid B normally contains an organic solvent such as methylene chloride as a foaming agent. When such a foaming raw material is poured into the vessel of expanded polystyrene, the organic solvent, typically methylene chloride can dissolve the vessel of expanded polystyrene in a moment so that the foaming liquid will run off the vessel, spoiling the vessel's purpose of separating liquid B from liquid A. Even if the foaming liquid does not run off, the organic solvent will soften the vessel to such an extent that the vessel may not retain its shape until the end of foaming. In either case, paddings cannot be molded in a stable manner.
In ordinary part molding procedures, a source material is introduced into the mold through a nozzle which is cleaned with an organic solvent such as methylene chloride on every shot of the source material. The organic solvent used for the cleaning purpose can impair the vessel.
(2) The method in question includes the step of thermally contracting the vessel. The thermally contracted vessel preferably has the same flexibility as polyurethane foam. However, an expanded polystyrene which is formulated to provide such flexibility after thermal contraction is low in strength and less self-sustaining before thermal contraction so that vessels formed therefrom will often fail to retain their shape upon pouring of foaming liquid. In turn, if vessels are formed from an expanded polystyrene which is formulated to be self-sustaining upon pouring of foaming liquid therein and during foaming, they can become too hard and brittle through thermal contraction. Such hard and brittle vessels will injure flexibility and elasticity required for paddings and tend to crack or fracture under an external force or deformation.